A. Field of the Invention
The present invention generally relates to electrodes for implantable cardiac stimulators, particularly defibrillators employing a xe2x80x9chot canxe2x80x9d stimulus generator housing. More particularly, the invention relates to such stimulation electrodes having a coating that protects the electrode surface from oxidation, and still more particularly to such oxidation resistant coatings that contain a conduction-enhancing medium.
B. Description of the Related Art
Abnormal rhythms or arrhythmias can arise in the heart as a consequence of an impairment of the heart""s electro-physiologic properties. Tachycardia, for example, is an arrhythmia characterized by rapid beating of the affected cardiac chamber which, in some instances, may lead to fibrillation. In other instances, fibrillation may arise in a diseased heart without the advance episode of tachycardia.
During fibrillation, sections of conductive cardiac tissue of the affected chamber undergo completely uncoordinated, random contractions, quickly resulting in a loss of the blood-pumping capability of that chamber. During ventricular fibrillation (i.e., fibrillation occurring in a ventricle), cardiac output ceases instantaneously. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood, and death of the patient will occur within minutes.
Because ventricular fibrillation is frequently caused by ventricular tachycardia, various methods and devices have been proposed to treat and arrest the tachycardia before the onset of fibrillation. Conventional techniques for terminating tachycardia include pacing therapy and cardioversion. In the latter technique, the heart is shocked with one or more current or voltage pulses of considerably higher energy content than is delivered in pacing pulses from a pacemaker. Unfortunately, cardioversion itself presents a considerable risk of precipitating fibrillation, as a result commonly called xe2x80x9crefibrillation.xe2x80x9d
Defibrillation, that is, the method employed to terminate fibrillation, generally involves applying one or more high energy xe2x80x9ccountershocksxe2x80x9d to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections, thereby restoring the synchronized contractions of the atria and ventricles. Successful defibrillation requires the delivery to the heart of the afflicted person an electrical pulse containing energy at least adequate to terminate the fibrillation and to preclude refibrillation. Although high intensity defibrillation shocks are often successful in arresting fibrillation, they tend to precipitate cardiac arrhythmias, which themselves may accelerate into fibrillation, i.e., refibrillation. Additionally, high intensity shocks can cause permanent injury to the lining of the heart (myocardium).
In the conventional approach of external defibrillation, conducting paddles or electrodes are positioned on the patient""s chest and electrical energy typically in the range of 100 to 400 joules is delivered to the chest area in the region of the heart. When fibrillation occurs during open heart surgery, internal paddles may be applied to opposite surfaces of the ventricular myocardium, and the energy required for defibrillation is considerably less, on the order of 20 to 40 joules.
More recently, implantable defibrillators have been developed for use in detecting and automatically treating ventricular fibrillation. In the last twenty years, a vast number of improvements in implantable defibrillators, including fibrillation detectors and high energy pulse generators with related electrode configurations, have been reported in the scientific literature and disclosed in patent publications.
Typically, electrodes for implantable defibrillators are made similarly to those developed for cardiac pacemakers, except defibrillation electrodes are larger than those used for cardiac pacing because a greater area of the heart tissue needs to be stimulated. These electrodes may be in the form of patches applied directly to the heart. The most common approach in the past has been to suture two patches to the epicardial tissue via thoracotomy. It has been theorized that electrodes with large surface areas are important for a wider distribution of current flow and a more uniform voltage gradient over the ventricles. Others have postulated that uniformity of current density is important since: (i) low gradient areas contribute to the continuation or reinitiation of ventricular fibrillation, and (ii) high current areas may induce temporary damage, that then may cause sensing difficulties, set-up areas of reinitiation of fibrillation, or even potentially cause permanent damage (new arrhythmias, decreased contractility, and myocardial necrosis).
For most patients, the best conventional devices and implantation methods are those that avoid surgical entry into the chest cavity and attachment of epicardial electrodes. Employing a less invasive surgical technique, one or more defibrillation electrodes are implanted proximate the pleural cavity and rib cage, and are used in combination with one or more coil electrodes positioned in the right atrium or right ventricle of the heart. This kind of defibrillator is described in U.S. Pat. No. 5,203,348, issued to Dahl, et al.
Stimulation of tissues requires that the charge be injected reversibly by a purely capacitive mechanism. In such a mechanism, the electrode behaves as a charge flow transducer whereby electrical discharge takes place in a uni-directional manner between media exhibiting different charge flow properties. The capacitive mechanism allows electrons to flow away from the stimulation electrode causing electrical charges at the electrode/electrolyte interface to orient themselves in order to cause a displacement current through the electrolyte. Since the electrolyte is an ionic medium, the slight displacement of the ions in reorientation creates a charge flow.
When irreversible chemical reactions begin to occur as a result of poor electrode selection or exceedingly high currents or other thermodynamic or kinetic limitations, the mechanism is no longer capacitive. Irreversible faradaic reactions may lead to water electrolysis, oxidation of soluble species, and metal dissolution from the electrode. In addition, some of the products of the reactions may be toxic. Neither gas evolution nor oxide formation reactions contribute to electrical stimulation of excitable tissue and stimulation energy is wasted in electrolyzing the aqueous phase of blood instead of carrying desirable charged species from one electrode to the other via the tissues. Because of the need for high energies in defibrillating the heart, higher currents are usually generated from the defibrillator than from a pacemaker. Under such circumstances, the efficiency of the electrode during high current generation is vital in not only reducing the defibrillation threshold, but in reducing the unwanted gas reactions or oxide formation. Excessive levels of gassing reactions may also lead to embolism in other vital organs such as the brain. Thus, stimulation electrodes should preferably allow a large charge flow across the electrode-tissue interface without the risk of irreversible faradaic reactions. Selection and design of the metal of the electrode is critical.
A metal of choice in electrode manufacturing has traditionally been titanium. On a fresh titanium surface, however, oxygen ions react with the titanium anode to form an oxide layer. Once a finite oxide thickness has been formed on the surface, polarization increases further. A point is reached when the oxygen ions reaching the surface of the titanium cannot be reduced further to form the oxide, and instead are reduced to elemental oxygen to form oxygen gas. The oxide film developed on the surface of a titanium electrode, either naturally or electrochemically, is irreversible. It cannot be reduced to the original metal by passing a charge in the reverse direction. Hence, it is clear that virgin titanium metal is a poor choice for electrode construction, since it forms a semi-conductive oxide on its surface before and even during the electrical stimulation. Platinum, and much more so stainless steel, have been shown to undergo irreversible dissolution during stimulation as well.
Very recently, the xe2x80x9chot canxe2x80x9d type of implantable defibrillator has come into wide use. In this type of defibrillator, one of the defibrillation electrodes is formed by the stimulator housing itself, or a face, window or portion thereof. The unit is placed subcutaneously or extra-pericardially, such that the shock current will flow through the ventricular septum to a transvenous lead placed inside the heart. An example of this kind of defibrillator is described in U.S. Pat. No. 5,480,416 issued to Garcia et al.
Conventional titanium hot can electrodes, by their failure to be essentially reversible in redox reaction along their surfaces, permit build up of the irreversible electrochemical products upon the surfaces. This results in entrapment of ions, molecules, etc. derived from the serum or body tissue in closest contact with the electrode surface, such as chronic coagulation and fibrotic growth. This in turn results in a greater likelihood of coagulation of blood, fibrin formation and other clotting cascade moieties immediately next to the surface or entrapped therein. These entrapped and surface blocking particles further reduce the ability of the surface to pass a charge and lead to increased impedance across the electrode surface.
Titanium oxidation reactions are several times more likely in an oxidative environment than those of platinum or platinum alloys, but a thousand times less so than those of stainless steel. Unfortunately, due to the expense of platinum metal and the requirement for large amounts of metal in patch-type electrodes, production costs are too high for platinum electrodes. Because of the low current requirements for pacemaker electrodes, the need for high surface area may not be too critical. However, it becomes crucial to implement a high surface area electrode with low polarization so as not to cause too much gassing reactions. At this time, platinum is generally used only in pacemaker electrodes and transvenous defibrillation electrodes, and not in hot cans. The use of platinum in transvenous defibrillation electrode is either in the form of a planar, low surface area alloy as platinum/iridium, or as a smooth, low surface area platinum coating on a titanium substrate. In such latter cases, the use of maximizing the surface area by making the platinum porous is usually avoided because of the possibility of increased clotting on a porous surface. However, this exposes the possibility of increased current density and increased gassing reaction which may lead to possible embolism. Thus, even though oxidation problems are more prevalent in them, titanium is typically used for conventional pacing and defibrillator electrodes as the substrate material.
U.S. Pat. No. 5,601,607, issued to Adams, also discusses the problem with achieving optimal electrode function with conventional hot can, also called xe2x80x9cactive can,xe2x80x9d electrode designs due to oxidation of the can material, particularly with cans made of titanium or stainless steel. The ""607 patent addresses the problem by coating the can with a noble metal, such as platinum, to reduce the effects of oxidation on the can material over multiple shocks.
A problem that is still not widely recognized at this time by those working in the area of cardiac stimulators, however, is that the housing (xe2x80x9ccanxe2x80x9d) material becomes porous as increasing numbers of shocks are administered, particularly with conventional titanium cans. As a result, the interfacial contact between the can surface and the body tissue diminishes and the path for adequate delivery of ionic current is correspondingly reduced, as illustrated in FIGS. 1A-B, and discussed in the Detailed Description, below. This results in high polarization at the can/tissue interface and leads to unacceptably high levels of defibrillation thresholds (DFTs).
In order to avoid passivation on the surface of titanium pacer electrodes, an iridium oxide (xe2x80x9cIrOxxe2x80x9d) coating has been employed on the electrode of a pacing lead, as disclosed in U.S. Pat. No. 4,919,135 (Phillips, Jr. et al.), issued to Intermedics, Inc. The oxide formed by iridium is very stable, does not grow further, and is electrically conductive. In addition, it provides protection for the underlying metal and is reversible to aqueous based redox species, undergoing reversible redox reactions with species such as hydrogen ions and hydroxyl ions, leading to the formation of higher oxidation state surface oxides. U.S. Pat. Nos. 4,717,581 and 4,677,989 teach deposition of iridium oxide onto the metal surface of an electrode. IrOx is rough, however, and it has been observed that rough-surfaced electrodes usually tend to cause scar tissue formation. In a surprising finding using the electrodes of the invention, it was found that the electrodes are capable of reducing the amount of both acute and chronic coagulation of blood surrounding the electrode. It is postulated that this reduction in the amount of coagulation of blood is a direct result of the reversible reduction-oxidation occurring over the enhanced electrically-accessible area of the electrodes. Where coagulation occurs immediately upon placement of the electrode in the tissue, it is said to be acute. Certain prior art electrodes have failed to be essentially reversible in redox reaction along their surfaces where the build up of the irreversible electrochemical products upon the surface results in entrapment of ions, molecules, etc. derived from the serum or tissue in closest contact with the electrode surface (chronic coagulation, fibrotic growth). This in turn results in a greater likelihood of coagulation of blood, fibrin formation and other clotting cascade moieties immediately next to the surface or entrapped therein. IrOx coated electrodes have not been employed for hot can electrodes.
An electrically stable can material that does not form an oxide in situ when subjected to numerous shocks, and provides a large charge flow, is needed for construction of the hot can defibrillator electrode. Such a material should also be very conductive and provide a very good tissue/can interface. An ideal electrode with a large charge flow can be obtained by selecting materials that: (a) are electrically stable; (b) undergo reduced unwanted reactions such as oxide formation or gassing reactions; (c) reversible; (d) reduce inflammation of the adjacent tissue and hence, provide thinner fibrous capsule at the electrode/tissue interface; (e) provide a continuum electrical interface with the tissue and electrode; and (f) are designed for high surface area. It would be beneficial to have a hot can defibrillator electrode that maintains a uniform defibrillation threshold over the useful lifetime of the unit.
The compositions, apparatus and methods of the present invention provide a way to remedy some of the disadvantages of conventional hot can defibrillator electrodes by coating the housing (xe2x80x9ccanxe2x80x9d) of an implantable cardiac stimulator with material that maintains a surface that is stable and does not undergo significant change during repetitive shocks over the lifetime of the device. The new ionically conductive polymer coatings provide a corrosion resistant barrier for the metal housing and the ionic carrier itself is able to undergo reversible redox reactions. The reversible redox capabilities of the ionic carrier is desirable so as to prevent a continuous build-up of species such as irreversible oxides or other compounds that may degrade undesirably over time and could lead to poor transport barriers and high impedance at the can/tissue interface. The use of coatings such as IrOx that provides a structure for reversible ion transport helps to avoid such problems associated with build-up of undesirable species. In addition, the reversible characteristic provides added redox capacity to the charge stimulation which aids in reducing the current density and distribution of charge over a large surface area, minimizing tissue ingrowth and tissue irritations as well as reducing gassing reactions that may lead to embolism. Plain titanium electrodes behave in the opposite manner to IrOx coated titanium electrodes.
Also comprehended by the present invention is a method of making an electrode having a roughened, or porous, platinum surface over a titanium substrate, and an ionically conductive polymer coating filling in the pits and crevices of the platinum surface to retain the effective high conductive surface area while providing a smooth, flat tissue/can interface. Advantages of employing a hot can having the new enhanced surface/polymer coated electrode, compared to conventional titanium cans, arise from having better interfacial contact between the polymer/platinum and the tissue. Therefore, the loss of energy that would otherwise take place due to poor interfacial conductivity is minimized. This additionally results in low polarization characteristics, as impedance that typically results from voids in a dry pocket or voids due to the porous electrode can is reduced with the hot cans prepared as described herein.
Since the platinum layer of certain preferred embodiments of the present invention is applied as a very thin layer on the etched titanium, the resulting electrode""s surface roughness is still appreciable. In order to maximize electrode performance, a polymeric coating is then applied to the platinum so that the outer surface is fairly smooth. A smooth surface optimizes the interface between the electrode and the fibrous capsule which almost always develops in vivo over a period of time. During implantation, injection of saline within the fibrous pocket allows the saline to absorb into the polymer and enhance and maintain a very good ionically conducting interface, thereby eliminating the dry pocket syndrome which otherwise results on a porous electrode surface. Use of a cardiac stimulus generator having the smooth polymeric interface of the present invention will result in a thinner fibrous capsule formation and enhances the opportunity for blood channel development between the electrode and the bulk of the tissues. This, in turn, will allow more efficient conduction to the right ventricular (xe2x80x9cRVxe2x80x9d) electrode.
The hot can electrode of the present invention is able to undergo essentially freely and completely reversible reduction-oxidation across the surface of the electrode. Since this renewal process is reinitiated upon each pulse or charge delivery, there is a much greater active surface life for the hot can electrode of the invention over those previously known. Although preferred embodiments of the invention describe a xe2x80x9chot canxe2x80x9d type of electrode, the coatings and methods of the present invention may also be used to make any other stimulus electrode with a porous structure, such as RV and superior vena cava (xe2x80x9cSVCxe2x80x9d) electrodes.
In accordance with the present invention, an ionically conductive polymeric composition for coating an implantable cardiac stimulus electrode is provided. The composition comprises a polymer such as polyethylene oxide, polyethylene terpthalate, a hydrogel or a polyacrylate, mixed together with an ionic medium such as NaCl. The molecular weight of the polymer is large enough to avoid solubilization of the polymer or the ionic medium when an electrode coated with the polymer composition is used in the body for delivering electrical pulses. Preferably the molecular weight is about 100,000 to 10,000,000 daltons, and more preferably between 100,000 to 5,000,000. The preferred ionic medium is NaCl, but other similarly ionizable compounds may be substituted, as long as it does not significantly alter the recipient""s body chemistry during the period of time that the coated device will be in place. In some embodiments, the polymeric composition also includes a steroid and/or an inorganic filler material. In other embodiments, the polymeric coating includes an antithrombotic, anticoagulant, antiseptic or anti-infectant or a thrombolytic agent to enhance the efficiency of the unit in situ or to provide other localized beneficial medicaments in long-lasting form.
Another embodiment of the invention provides an electrode for a cardiac stimulator. The electrode comprises a titanium substrate that has a porous surface structure, and that is electrically connected to a stimulus generator. Minimally coating the porous structure is a layer of an oxidation resistant metal. The metal coated porous structure is filled with an electrically conductive polymeric coating and the polymeric coating also forms a smooth outer surface on the electrode. The metal that is used to coat the porous structure is platinum, ruthenium, rhodium, palladium, osmium, iridium or an alloy of those metals.
An alternative embodiment of the invention provides an electrode for a cardiac stimulator that has a titanium substrate with a porous electrically-conductive metal or metal oxide layer deposited on top of the titanium substrate to form a porous layer. Coating and permeating the porous layer is an electrically conductive polymeric coating which provides a smooth outer surface on electrode. The deposited porous layer may be platinum black, or it may be a metal oxide of platinum, ruthenium, rhodium, palladium, osmium or iridium. In certain embodiments, the porous layer is IrOx.
Also provided by the present invention is an implantable cardiac stimulator comprising an electrical stimulus generator capable of delivering a defibrillation shock and having a housing that encloses the stimulus generator. At least a portion of the housing serves as a first electrode, which is electrically connected to the stimulus generator. The first electrode includes a substrate, such as titanium, that has a porous surface, and the first electrode also includes an outer surface. A second electrode is also electrically connected to the stimulus generator, and this second electrode is adapted for placement inside the heart. The second electrode and the first electrode cooperate with the stimulus generator in delivering defibrillation shocks to the heart. In this embodiment, a layer of oxidation resistant metal minimally covers the pores of the first electrode and an electrically conductive polymeric coating permeates the metal-covered pores and forms the smooth outer surface of the first electrode. In some embodiments, the cardiac stimulator also has an electrically insulative coating with a window in the coating.
In still another embodiment of the present invention, a method of making an implantable cardiac stimulator is provided. The method includes removing entrapped gas from the porous surface of a stimulator housing, at least a portion of the housing forming an electrode with a porous-surfaced metal substrate. The pores are then impregnated with a solution comprising a biocompatible polymer, and a biocompatible ionic carrier, followed by evaporation of the solvent from the impregnated surface. A smooth polymeric outer surface is thereby formed on the electrode. Suitable polymers include polyethylene oxide, polyethylene terpthalate, polyacrylates, and the like. Impregnation of the pores can be accomplished by soaking or ultrasonicating the porous surface in a solution of 5-10% polyethylene oxide and 1-2% NaCl in alcohol.
Another embodiment of the invention is an improved method of making an implantable cardiac stimulator of the xe2x80x9chot canxe2x80x9d type, having a titanium housing. The new method provides for applying a porous coating of a noble metal or an oxide thereof, such as platinum or iridium oxide. The porous coating may also be platinum, ruthenium, rhodium, palladium, osmium, iridium, or an oxide thereof. An ionically conductive biocompatible polymer, which is capable of reversible reduction-oxidation, is applied to the porous coating such that it permeates and covers the porous coat and forms a smooth outer surface on the electrode.
In an alternative embodiment, an improved method is provided for making an implantable cardiac stimulator having a titanium housing comprising one of the stimulation electrodes, wherein the xe2x80x9chot canxe2x80x9d electrode has a porous structured surface. A coating of a metal such as platinum, ruthenium, rhodium, palladium, osmium and iridium is applied to the porous electrode surface is such a way that the metal coating essentially conforms to and retains the porous structure. To the metal-coated porous electrode, an ionically conductive biocompatible polymer capable of reversible reduction-oxidation is applied so as to fill and cover the porous structured electrode, whereby a smooth outer surface is formed. This surface is then coated with an ionically conductive polymeric material coating and filling the pores of the electrode. Preferably, the coating material is biocompatible, chemically and mechanically stable over several years and does not dissolve or leach out over time. The outer surface of the preferred electrode of the present invention preferably includes a smooth coating of the conductive material so that the interface with the tissue is complete, permitting the entire surface area of the electrode to be utilized and eliminating voids that would otherwise cause the DFT to increase with time and use.
One embodiment of the present invention provides an ionically conductive polymeric material for coating an implantable porous metal or metal oxide electrode, the polymeric material containing a polymer and an ionic medium, or electrically conductive carrier, admixed therewith. The polymer preferably has a molecular weight large enough to avoid solubilization or leaching of the polymer or the ionic medium when an electrode coated with the polymeric material is used for its intended purpose, such as for delivering a defibrillation shock to the heart. The polymeric material may be polyethylene oxide, polyethylene terpthalate, polyacrylates, or other suitable polymeric hosts that satisfies the need as a biocompatible medium capable of maintaining an ionic carrier within the polymeric matrix. The ionic carrier may be NaCl, or another similarly ionizable compound that is biocompatible with the body and does not significantly alter the body chemistry over long periods of time.
Another embodiment of the present invention provides an implantable cardiac stimulator having an electrical stimulus generator capable of delivering a defibrillation shock. The housing enclosing the stimulus generator has at least a portion of its outer surface serving as an electrode in electrical communication with the stimulus generator, and has an electrically conductive polymeric coating on at least that portion of the housing that serves as the electrode. The defibrillator also has a second electrode that is electrically connected to the stimulus generator and is made to be placed in direct contact with the heart, by routing through an artery, for example. This second electrode is capable of cooperating with the can electrode on the stimulus generator housing to deliver a defibrillation shock to the heart, when needed. The conductive polymeric coating on the outside of the housing preferably deters oxidation of the metal or metal oxide surface of the housing, even when the outer surface of the housing is a porous metal oxide, and the ionically conductive coating itself is oxidation resistant. Preferably, the defibrillator is adapted for implantation pectorally.
In certain preferred embodiments of the defibrillator of the present invention, the housing is titanium and the outer surface, or a part or area thereof, has a first coating of IrOx or another surface enhanced platinum or other noble metal, or oxide of such metal. This first coating is permeated and covered by an electroconductive polymeric coating such as polyethylene oxide having a molecular weight of about 100,000 to 5,000,000 daltons.
The present invention also provides a method of making the defibrillator described above. The method includes removing entrapped gas from the porous surface of the housing, impregnating the porous surface with a solution comprising 5-10% polyethylene oxide and 1-2% NaCl in a suitable solvent such as alcohol, and evaporating the alcohol from the impregnated surface to form a stable coating throughout and on top of the porous layer. The porous layer may be saturated with the electroconductive polymeric solution by soaking or untrasonicating the apparatus so as to completely fill or wick the porous body, or the porous layer may be soaked with a precursor polymer solution and radiation, heat or chemically cured subsequently yielding the same result.
The present invention also provides an improved method of making an implantable cardiac defibrillator having a housing comprising one of the electrodes, said electrode having a metal or metal oxide surface, wherein the improvement consists of coating the surface with an electrically conductive polymer capable of reversible reduction-oxidation.
These and other objects, features and advantages of the present invention will become apparent with reference to the following description and drawings.